1. Field of the Invention
The present invention relates generally to organic positive temperature coefficient (PTC) thermistors used as face-like heating devices, and more particularly, to an organic positive temperature coefficient thermistor which includes an improvement in the structure of electrodes formed on a sheet exhibiting a positive temperature characteristic of the resistance.
2. Description of the Prior Art
For example, a material obtained by thoroughly mixing a polyolefin such as polyethylene with conductive particles such as carbon black, metal powder or carbon graphite exhibits a positive temperature characteristic of resistance. An organic positive temperature coefficient thermistor using a sheet made of this material can be used as a flexible face-like heating device.
An example of the above described organic positive temperature coefficient thermistor is shown in FIG. 1. In an organic positive temperature coefficient thermistor 1, comb-shaped electrodes 3 and 4 are formed on one surface of a sheet 2 obtained by dispersing conductive particles in an organic polymer material such as polyolefin. The comb-shaped electrodes 3 and 4 respectively comprise power feeding electrodes 3a and 4a extending along side edges of the sheet 2 and a plurality of conductive portions 3b and 4b extending from the power feeding electrodes 3a and 4a toward the respective other power feeding electrodes 4a and 3a. The plurality of conductive portions 3b and 4b are arranged so as to be interdigitated.
In the organic positive temperature coefficient thermistor 1 shown in FIG. 1, heat is uniformly generated to some extent in the region where the plurality of conductive portions 3b and 4b are interdigitated. However, the region where the power feeding electrodes 3a and 4a are arranged along the side edges of the sheet 2 hardly contributes to heat generation. Consequently, heat cannot be efficiently generated in the entire sheet 2, thereby making it impossible to obtain satisfactory thermal efficiency.
Another organic positive temperature coefficient thermistor, having a structure in which an electrode is formed over both surfaces of a sheet 2 shown in FIG. 1, is known, but is not illustrated herein. In this organic positive temperature coefficient thermistor, heat is uniformly generated on the whole surface of the sheet 2. Consequently, this organic positive temperature coefficient thermistor is superior in thermal efficiency to the positive temperature coefficient thermistor shown in FIG. 1. However, if the whole surface electrode is formed of, for example, a metal foil, there are differences in the coefficient of thermal expansion and in flexibility between the sheet and the metal foil. Accordingly, the flexibility which is an advantage of the organic positive temperature coefficient thermistor is lost.
Furthermore, if the whole surface electrode is formed of a conductive paste such as an Ag paste, the organic positive temperature coefficient thermistor is substantially increased in cost, although it retains its flexibility.
Another problem is that in the organic positive temperature coefficient thermistor with the whole surface electrode, the resistance value of the whole thereof becomes lower than that of the organic positive temperature coefficient thermistor in which the comb-shaped electrodes 3 and 4 are formed. Accordingly, the specific resistance of the sheet must be increased by approximately tenfold to hundredfold that of the organic positive temperature coefficient thermistor in which the comb-shaped electrodes are formed. If the specific resistance of the organic positive temperature coefficient thermistor is thus increased, however, the stability of the specific resistance is lost, resulting in increased variation in specific resistance. As a result, the organic positive temperature coefficient thermistor varies very greatly in characteristics depending on the product.